Just a quick thought, what might not be a bad idea is if you were able to run a wire (after the relay) from each compressor to a LED or small lamp inside the van. This would mean you could monitor the compressors behaviour.
I had dual 480c’s for my K3’s and I had a relay fail on me whilst driving along, I only new it had failed because the indicator lamp I wired to it was still on 5 mins later after the other compressor & electronic pressure gauge was still gradually going up!
Relay failure is quite a common thing, I had a second relay failure a few months later & one of the compressors cut in & kept pumping away over night till the battery went dead & the electronic gauge was somehow (despite having a pressure release valve) reading 320psi in a 8.5 gallon tank!
I then fitted a whole array of kill switches. 
I really like how everything came out. Nice work!
You could have possibly used a 90 degree fitting and had a “remote” drain… I used to press a button to drain the water that was in the tank from inside the cab…
Hi all
Latest mod fitted an air line connecter to my tank tonight
Bought a coiled 25ft airline so all ready for this weekend trip away and best of all is i can now take the pool and blow it up with complete ease 
Or Burst it!!! pmsl!!:eek: lol